Lithium ion secondary batteries including a lithium ion secondary battery have a high specific energy density. As such, the development of these batteries for various purposes is expected, including the usage for electric vehicles and electricity storage.
For the various purposes, lithium ion secondary batteries are required to have a large output. A lithium ion secondary battery having a large capacity and output in the early stage may deteriorate in the capacity and the output because DC resistance (DCR) of the battery increases during the course of subsequent charge/discharge cycles or with the passage of time, and a system including such a lithium ion secondary battery cannot continue the operation in some cases. A cathode including an oxide mainly containing manganese has a prominent tendency of the DCR increase at the cathode related to Mn elution from the Mn oxide.
To avoid such a problem, a method has been examined to form a protective film on the cathode surface (Patents Literatures 1 to 6). Patent Literature 1 discloses a technique of treating a LiMn2O4 cathode active material with boric-acid, thus covering the surface with boron. Patent Literature 2 relates to the invention of forming a first cover layer containing a boron oxide and a second cover layer containing a lithium-cobalt composite oxide on the surface of a lithium-manganese oxide. Patent Literature 3 discloses a method of forming a surface treatment layer containing elements such as boron on the surface of a cathode active material such as a lithium-containing metal oxide. Patent Literature 4 discloses a technique of coating the surface of a cathode with alkali metal, alkali earth metal or the like. Patent Literature 5 relates to the invention of adding a coating formation compound having a carbon-carbon unsaturated bond in a molecule and having a single bond or a double bond between 13-group, 14-group or 15-group element M and oxygen (O), thus suppressing a decomposition reaction of electrolytic solution on the cathode. Patent Literature 6 discloses a method of forming a coating layer on the surface of a cathode active material using an amphoteric compound containing zinc or the like.
Additionally, a method for improved storage properties at a high temperature has been found by mixing two types of electrolytes. Patent Literatures 7 to 9 disclose techniques for improved storage properties at a high temperature using electrolytic solution including the mixture of LiBF4 and LiPF6.